A magneto-optical recording medium whereon a direct overwrite is permitted by the light intensity modulation method is provided with a plurality of magnetic layers being laminated wherein an exchange coupling exists between the adjacent magnetic layers. The above magneto-optical recording medium has been viewed with interest for high speed data transfer, and has been earnestly studied. For a magnetic substance of the magnetic layers, rare-earth transition metal alloys (hereinafter referred to as RE-TM) having a perpendicular magnetic anisotropy are known.
For the magneto-optical recording medium, a magneto-optical disk with RE-TM quadrilayered films has been known (see J. Appl. Phys. Vol.67(1990), FUKAMI et al. pp.4415-4416 published by American Institute of Physics).
The above magneto-optical disk does not require an initializing magnet. This advantage can be taken to miniaturize the device. As shown in FIG. 6, a magneto-optical disk 20 includes a transparent substrate 21 whereon a first magnetic layer 22, a second magnetic layer 23, a third magnetic layer 24, and a fourth magnetic layer 25 are laminated in this order.
The magneto-optical disk 20 has an exchange coupling in the adjacent magnetic layers. Additionally, respective Curie temperatures Tci of the magnetic layers 22, 23, 24, and 25 (i=1, 2, 3, or 4 indicating the ordinal number of the magnetic layer) satisfy the following inequation. EQU Tc.sub.3 &lt;Tc.sub.1 &lt;Tc.sub.2, Tc.sub.4 (1)
The first magnetic layer 22 serves as a memory layer for recording thereon information using the magnetization direction. On the other hand, neither of the second magnetic layer 23, the third magnetic layer 24, and the fourth magnetic layer 25 has a function as a carrier of the information. These layers are provided so as to enable the direct overwrite by the light intensity modulation method.
The second magnetic layer 23 is used for recording through a high temperature process, and it is also used for initializing through a low temperature process.
The third magnetic layer 24 serves as a switching layer for switching off the exchange coupling from the fourth magnetic layer 25 in the high temperature process.
The fourth magnetic layer 25 is arranged such that Tc.sub.4 thereof is set above the temperature range in which an overwrite is permitted, so that the magnetization direction of the fourth magnetic layer 25 is not reversed. At room temperature, the sublattice magnetization direction of the second magnetic layer 23 is arranged in that of the fourth magnetic layer 25 by the exchange coupling through the third magnetic layer 24, thereby initializing the second magnetic layer 23.
The direction of the fourth magnetic layer 25 is arranged in one direction (for example, upward), and the Curie temperature thereof is set the highest. Therefore, even when the temperature of the fourth magnetic layer 25 is raised by projecting the laser beam, the magnetization direction thereof will not be reversed. Additionally, the above magnetization is the summation of the respective sublattice magnetizations of the rare-earth metal and the transition metal.
The overwrite process is as follows: The sublattice magnetization direction of the first magnetic layer 22 is arranged in the initialized sublattice magnetization direction of the fourth magnetic layer 25 by the exchange coupling in the low temperature process. On the other hand, the sublattice magnetization direction of the first magnetic layer 22 is arranged in the direction opposite to the sublattice magnetization direction of the fourth magnetic layer 25 by the external magnetic field Hex for recording (bias magnetic field) in the high temperature process. With the above arrangement, the overwrite is permitted on the first magnetic layer 22, thereby recording information.
The high temperature process is a recording process wherein after arranging the magnetization direction of the second magnetic layer 23 in the direction of the external magnetic field Hex for recording, the arranged magnetization direction is copied to the first magnetic layer 22 at a temperature between Tc.sub.3 and Tc.sub.1 (see FIG. 7). Here, the magnetization direction of the external magnetic field for recording Hex is set the direction opposite to the sublattice magnetization direction of the fourth magnetic layer 25.
More concretely, as shown in FIG. 7, in the case where the temperature of the magneto-optical disk 20 is raised to the vicinity of Tc.sub.2 by increasing the laser power, the direction of the sublattice magnetization of the fourth magnetic layer 25 is not changed since the temperature thereof is below Tc.sub.4. On the other hand, the magnetizations of the first magnetic layer 22 and the third magnetic layer 24 disappear as the temperatures thereof are respectively raised above Tc.sub.3 and Tc.sub.1.
Therefore, the magnetization of the second magnetic layer 23 is reversed in the direction of the external magnetic field Hex for recording without being affected by the exchange coupling from the first magnetic layer 22 nor from the third magnetic layer 24.
Thereafter, when the temperature of the magneto-optical disk 20 is dropped below Tc.sub.1, the magnetization direction of the second magnetic layer 23 is copied to the first magnetic layer 22. Further, when the temperature of the magneto-optical disk 20 is dropped below Tc.sub.3, the sublattice magnetization of the third magnetic layer 24 is arranged in the direction of the fourth magnetic layer 25 by the exchange coupling.
As the temperature of the magneto-optical disk 20 is further dropped, the sublattice magnetization of the second magnetic layer 23 is reversed in the direction of the fourth magnetic layer 25 by the exchange coupling through the third magnetic layer 24. Since the coercivity of the first magnetic layer 22 is already large in this state, the magnetization direction thereof is not affected by the reversed magnetization of the second magnetic layer 23.
Therefore, the sublattice magnetization direction of the first magnetic layer 22 can be maintained in the direction opposite to the sublattice magnetization direction of the fourth magnetic layer 25. The above recorded state of the first magnetic layer 22 is, for example, represented by "1" state.
On the other hand, in the low temperature process, as shown in FIG. 8, the sublattice magnetization direction of the second magnetic layer 23 arranged in the sublattice magnetization direction of the fourth magnetic layer 25 is copied to the first magnetic layer 22 by the exchange coupling from the fourth magnetic layer 25 through the third magnetic layer 24.
More concretely, even when the temperature of the second magnetic layer 23 is raised to the vicinity of Tc.sub.1 by increasing the laser power stronger than that used for reproducing, since Tc.sub.2 is set above Tc.sub.1, and thus the coercivity thereof remains large, the sublattice magnetization direction of the second magnetic layer 23 is not reversed by the external magnetic field for recording.
This means that the sublattice magnetization of the second magnetic layer 23 is copied to the first magnetic layer 22 by the exchange coupling. Thus, the sublattice magnetization direction of the first magnetic layer 22 is arranged in that of the fourth magnetic layer 25. The above state of the first magnetic layer 22 thus initialized is represented by, for example, "0" state.
Also in the low temperature process, even if the sublattice magnetization of the third magnetic layer 24 disappears as the temperature thereof is raised above Tc.sub.3, the sublattice magnetization direction thereof is arranged in that of the fourth magnetic layer 25 by the exchange coupling between the third and fourth magnetic layers 24 and 25 when the temperature thereof is below Tc.sub.3.
As described, the above arrangement enables the direct overwrite by the light intensity modulation method through the high temperature and low temperature processes.
However, the above arrangement has the following problem. As shown in FIGS. 9(a)(b), when the recorded states "0" and "1" of the first magnetic layer 22 are compared, the respective magnetic energy conditions are not equivalent. In the figures, a white arrow shows a magnetization, an arrow in the white arrow shows the sublattice magnetization direction of the transition metal or the rare-earth metal, an dotted line shows a high interface wall energy generated between the magnetic layers.
Namely, in the above arrangement, the energy condition between the first magnetic layer 22 and the second magnetic layer 23 is not equivalent, since a higher energy exists in the "1" state of the first magnetic layer 22 than the "0" state of the first magnetic layer 22 by an amount of an interface wall energy.
Therefore, the above arrangement of the magneto-optical disk 20 presents the problem that the recorded state of the first magnetic layer 22 becomes unstable by being reversed from the "1" state to the "0" state due to the temperature change during reproducing or storing or due to the application of an unexpected external magnetic field, etc. Thus, reliable recorded states cannot be obtained.